1. Field of the Invention
This invention relates to radiation therapy equipment for the treatment of tumors or the like and specifically to an improved method of characterizing the radiation beam of such systems and confirming the dose received by the patient using a portal image of radiation exiting the patient.
2. Background of the Invention
Medical equipment for radiation therapy treats tumorous tissue with high energy radiation. The dose and the placement of the dose must be accurately controlled to insure both that the tumor receives sufficient radiation to be destroyed, and that damage to the surrounding and adjacent non-tumorous tissue is minimized.
Internal-source radiation therapy places capsules of radioactive material inside the patient in proximity to the tumorous tissue. Dose and placement are accurately controlled by the physical positioning of the isotope. However, internal-source radiation therapy has the disadvantages of any surgically invasive procedure, including discomfort to the patient and risk of infection.
External-source radiation therapy uses a radiation source that is external to the patient, typically either a radioisotope, such as 60Co, or a high energy x-ray source, such as a linear accelerator. The external source produces a collimated beam directed into the patient to the tumor site. External-source radiation therapy avoids some of the problems of internal-source radiation therapy, but it undesirably and necessarily irradiates a significant volume of non-tumorous or healthy tissue in the path of the radiation beam along with the tumorous tissue.
The adverse effect of irradiating of healthy tissue may be reduced, while maintaining a given dose of radiation in the tumorous tissue, by projecting the external radiation beam into the patient at a variety of xe2x80x9cgantryxe2x80x9d angles with the beams converging on the tumor site. The particular volume elements of healthy tissue, along the path of the radiation beam, change, reducing the total dose to each such element of healthy tissue during the entire treatment.
The irradiation of healthy tissue also may be reduced by tightly collimating the radiation beam to the general cross section of the tumor taken perpendicular to the axis of the radiation beam. Numerous systems exist for producing such a circumferential collimation, some of which use multiple sliding shutters which, piecewise, may generate a radio-opaque mask of arbitrary outline.
The radiation beam may also be controlled by insertion of wedges or blocks into the beam to reduce the intensity or fluence of the beam by means of attenuation in some areas. U.S. Pat. No. 5,317,616 issued May 31, 1994, incorporated by reference and assigned to the same assignee as the present invention, describes a shutter system that provides an alternative to wedges for reducing fluence or intensity portions of the radiation beam by temporally modulating the radiation beam with collimator leaves.
The ability to selectively adjust intensity of individual portions or rays in the radiation beam is essential if the cumulative dose from multiple angles is to be accurately controlled. For example, two exposures along perpendicular directions may be made to expose a generally rectangular area within the patient. In order that the corner of the rectangular area closest to both of the radiation sources at the two angles does not receive a disproportionate radiation dose, wedges are used in each exposure to reduce the beam intensity on the side of the beam closest to the radiation source for the other exposure side. In this way, after the two exposures, a uniform dose has been received by the area.
In order to confirm the positioning of the collimation blades and blocks used in a particular radiation therapy session, a xe2x80x9cportal imagexe2x80x9d may be obtained in which radiation exiting from the patient is recorded on x-ray film or the like. A visual examination of this image provides a poor quality x-ray radiograph that gives a gross indication that the xe2x80x9cgeometryxe2x80x9d of the radiation beam is correct.
Confirmation of the dose received by the patient may be provided by the placement of point dosimeters on the surface of the body or within body cavities.
Developing a plan of radiation treatment involving radiation at multiple angles requires that the dose provided by the beam at each angle be well known. For this reason, it is normal practice to determine the dose distribution in a standard water phantom. The water phantom is generally a water-filled box placed within the radiation beam so that the beam cuts enters the box perpendicularly to the water surface. A small radiation detector, such as an ionization chamber, is physically scanned through the volume of the phantom to make dose measurements. Under the assumption that the phantom material is similar to a human patient, this dose distribution reflects that which may be expected in a human patient if the patient""s shape and actual tissue density are neglected.
The measurement of the dose distribution in this manner is time consuming and inaccurate. A change in collimation changes the intensity even along central and uncollimated rays as a result of scatter within the phantom and from the collimator itself. For this reason, a sequence of dose measurements must be made for different beam collimations and for the use of different blocks.
These dose distribution measurements also permit the evaluation of the uniformity of the radiation beam provided by the system, and in particular, the intensity in a cross-section of the radiation beam before it enters the patient (the xe2x80x9cbeam intensity profilexe2x80x9d). If the intensity of the beam deviates beyond a certain amount from a uniform value, adjustments may be made in the alignment of the radiation source and/or filters may be inserted into the radiation beam to improve its intensity profile. Because the beam intensity profile may vary over time, such measurements must be repeated on a regular basis.
Determining the dose distribution within a standard phantom for a variety of the beam sizes and filtrations by scanning a phantom with a dosimeter is time consuming and expensive.
The present invention provides a method of automatically characterizing the radiation beam of a radiation therapy system (and the dose to a phantom or patient) using the radiation intensity information contained within the portal image. The present invention recognizes that the amount of radiation exiting the patient or phantom, as provided by the portal image, together with knowledge about the composition and geometry of the patient or phantom, can be used to characterize the radiation beam intensity profile irradiating the patient or phantom without independent dose measurements by a scanning dosimeter.
This more accurate characterization of the radiation beam fluence profile may be used to generate other information including: 1) a more accurate determination of the dose received by the patient, 2) a scatter-free portal image that may better confirm the geometry of collimator blades and patient blocks, and 3) a verification that the correct patient volume is being treated.
Specifically, the present invention provides a method of calibrating a radiation therapy machine involving the steps of irradiating a phantom of known composition and geometry with a beam of radiation having a beam intensity profile and acquiring a measured portal image indicating the intensity of the beam of radiation along a number of rays and after it has passed through the phantom. An electronic computer receives the measured portal image as well as the character and composition of the phantom and models an expected portal image based on an irradiation of the phantom with an assumed beam intensity profile. This expected portal image is compared with the measured portal image to produce a correction image which increments or scales the assumed intensity profile in an iterative process. The process converges with the assumed intensity profile measurement equal to the actual intensity profile of the radiation therapy system.
Thus, it is one object of the invention to characterize the beam intensity profile of the radiation source without the need to measure the intensity of the beam prior to the patient or within a phantom.
The accurate characterization of the beam fluence profile may be used to determine a dose distribution throughout the phantom.
Thus, it is another object of the invention to eliminate the time consuming process of measuring dose within a water phantom with a movable dosimeter. Accurate modeling of the primary radiation passing through the phantom permits dose determinations for a variety of collimations and wedges with a single phantom measurement by including scatter contributions after the entire primary radiation has been determined.
The measure of the intensity profile may also be used to develop a simulated scatterless portal image based on modeled irradiation of a phantom without scatter.
Thus, it is another object of the invention to provide an improved version of the traditional portal image for geometric verification. Elimination of scatter produces a sharper image from which it is easier to verify that the patient has been properly treated.
The invention may also be employed where the phantom is replaced by a patient having had a CT scan of the irradiated volume which accurately characterizes the composition and geometry of that volume. In this case, an extremely accurate determination of the dose received by the patient may be obtained.